Add method/format information to =record-started

Add method/format information to =record-started

Eclipse CDT now supports enabling execution recording using two methods
(full and btrace) and both formats for btrace (bts and pt).  In the
event that recording is enabled behind the back of the GUI (by the user
on the command line, or a script), we need to know which method/format
are being used, so it can be correctly reflected in the interface.  This
patch adds this information to the =record-started async record.

Before:

  =record-started,thread-group="i1"

After:

  =record-started,thread-group="i1",method="btrace",format="bts"
  =record-started,thread-group="i1",method="btrace",format="pt"
  =record-started,thread-group="i1",method="full"

The "format" field is only present when the current method supports
multiple formats (only the btrace method as of now).

gdb/ChangeLog:

	* NEWS: Mention the new fields in =record-started.
	* common/btrace-common.h (btrace_format_short_string): New function
	declaration.
	* common/btrace-common.c (btrace_format_short_string): New
	function.
	* mi/mi-interp.c (mi_record_changed): Output method and format
	fields in the =record-started record.
	* record-btrace.c (record_btrace_open): Adapt record_changed
	notification.
	* record-full.c (record_full_open): Likewise.
	* record.c (cmd_record_stop): Likewise.

gdb/doc/ChangeLog:

	* gdb.texinfo (GDB/MI Async Records): Document method and
	format fields in =record-started.
	* observer.texi (record_changed): Add method and format
	parameters.

gdb/testsuite/ChangeLog:

	* gdb.mi/mi-record-changed.exp: Adjust =record-started output
	matching.